Spin rate compensator

ABSTRACT

Compensation for spin rate variation of an inertial measurement device comprising at least one translational acceleration transducer which is spun is achieved by passing the output signal from the translational acceleration transducer(s) through a circuit which has a frequency response inversely proportional to frequency.

BACKGROUND OF THE INVENTION

There are applications wherein both the output frequency and sensitivityof a measuring device are proportional to the rate at which the deviceis spinning. One class of such devices is disclosed in U.S. patentapplication Ser. No. 528,243 filed on Nov. 29, 1974 for "Apparatus forPerforming Inertial Measurements Using Translational AccelerationTransducers and for Calibrating Translational Acceleration Transducers"and assigned to the assignee of the present application. In thisapplication there is disclosed apparatus for measuring angular velocityby spinning one or more translational acceleration transducers, at leastone of which is placed parallel to the spin axis, spinning around anaxis perpendicular to the turn axis. The transducer output signal = K xturn rate x spin rate x cos (2πx spin rate x time) where K is a constantdepending upon system geometry and transducer sensitivity. It is thusseen that the output signal is a cosine wave of the same frequency asthe spin rate with an amplitude proportional to the turn rate multipliedby the spin rate. Thus, to achieve accurate turning angular ratemeasurements using such a device it has been necessary to know the spinrate by either maintaining the spin rate of the transducer constant orby independently measuring it and making suitable adjustment.

In many applications this may be no problem as the transducer can bespun using synchronous or other regulated motors. However, one of theobjects of the device is to provide inexpensively a gyroscopesubstitute. To do same inexpensively necessitates for some applicationsthe use of an inexpensive spin motor such as a spring wound or dcelectric motor. When such an unregulated motor is used, of course, thetransducer is not spun at a constant rate. Also gas driven or impulsestarted turn rate indicators for boost-glide missiles where the systemis given an initial excitation and then allowed to coast, thereby havingan unknown, changing spin rate, necessitates spin rate measurement.Measurement of the changing spin rate will increase the cost and incertain application may be impractical.

Also in the aforementioned patent application there is described anapplication using spinning translational acceleration transducers on aspinning projectile to make inertial measurements. In this applicationthe transducers are spun by the spinning projectile itself andtherefore, no additional spin motors are required. However, since aspinning projectile spins at a continuously decreasing rate during itstrajectory, it is necessary to know the instantaneous spin frequencythereof to make accurate measurements. This requires the use ofadditional transducers on the spinning projectile to make suchmeasurements or other spin measuring means. This can be both relativelyexpensive and difficult.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a spin ratecompensator.

It is another object of this invention to provide apparatus for makingthe sensitivity of a translational acceleration tranducer angular ratesensor independent of its spin rate.

It is a further object to provide a spin rate compensator simply,inexpensively and compactly.

Briefly, the sensitivity of a translational acceleration transducerangular rate sensor is made independent of its spin rate by passing theoutput signal from the transducer through a circuit which has afrequency response inversely proportional to frequency. The output fromsuch transducer is K x turn rate x spin rate x cos (2πx spin rate xtime) where K is a constant depending upon system geometry andtransducer sensitivity. From the above equation it is seen that theoutput signal is a cosine wave of the same frequency as the spin ratewith an amplitude proportional to the turn rate multiplied by the spinrate. By passing the output signal through a compensating circuit whosegain is inversely proportional to frequency, such as an integratorcircuit or a low pass RC filter in the region where the output isattenuated at the rate of 6 db per octave, the compensating circuitoutput is equal to K x turn rate x spin rate x 1/spin rate x sin (2πxspin rate x time). (The signal phasing shifts from cos to sin because ofthe integration circuit.) The spin rate cancels out of the amplitudeterm leaving the compensated output signal equal to K x turn rate x sin(2πx spin rate x time) which is at the spin frequency but the amplitudeof which is independent of spin rate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and other objects of thisinvention will become more apparent by reference to the followingdescripition taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 are typical waveforms of an uncompensated and a compensatedsignal;

FIGS. 2A-2C are curves illustrating the theory of operation of theinvention;

FIG. 3 is an illustration of one embodiment of the invention; and

FIG. 4 is an illustration of another embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Waveform A of FIG. 1 illustrates a typical signal from a spinning devicewhich is both amplitude and frequency dependent upon the spin rate ofthe device. Though the turn rate is constant, when the spin ratedecreases the amplitude and frequency also decrease. This characteristicof a transducer output signal is represented by the curve of FIG. 2Awhere it is shown that the amplitude of the transducer output signal isdirectly proportional to the spin rate of the transducer, though theturn rate is constant.

Compensation for change in spin rate of such a transducer is achieved byemploying the circuit shown in FIG. 3 wherein the output from atransducer 10 is passed through an integrator 12. Integrator 12 is, forexample, a conventional integrator comprised of both passive and activecomponents, for example, an operational amplifier.

A simplified passive integrator or low pass RC circuit is shown in FIG.4 wherein the output of transducer 10 is passed through an integrator 14comprising a series resistor 16 and a shunt capacitor 18. The gainversus frequency of RC circuit 14 is shown in FIG. 2B. This is a lowpass filter wherein the output thereof is substantially constant for apredetermined frequency portion and then drops at a 6 dB per octaverate. The sloping portion of the curve of FIG. 2B represents theoperating range of integrator 14 contrasted to conventional low-passfilters wherein the operating range is the constant amplitude portion ofthe curve. The output from transducer 10 = K x turn rate x spin rate xcos (2πx spin rate x time). By passing the output signal from transducer10 through either circuit 12 or 14 whose gain is 1/spin rate (inverselyproportional to frequency) in the region of interest, amplitudeindependent of frequency is achieved. The output from the compensatingcircuit = K x turn rate x spin rate x 1/spin rate x sin (2πx spin rate xtime). The spin rate cancels out of the amplitude term leaving thecompensated output signal = K x turn rate x sin (2πx spin rate x time)which is at the spin frequency but whose amplitude is independent ofspin rate in the region of interest as illustrated by FIG. 2C. A typicaloutput from such a compensating circuit is shown by waveform B of FIG. 1wherein the frequency of the signal decreases with the spin ratehowever, the amplitude is independent thereof.

As mentioned above, normally, the characteristics of the circuit 14would be selected to provide operation in the flat region or pass bandof the low-pass filter. However, Applicant operates in the attenuatingor sloping region thereby achieving spin rate compensation. The lowpass-band of the filter also enhances the signal by filtering out anyhigh frequency noise such as that produced by ball bearings.

The normal pass-band of the low pass filter is selected to be below thelowest frequency of interest which is just the reverse of the regularuse of low-pass filters where the drop in the pass-band of the filter isselected to be above the highest frequency of interest.

While I have described above the principles of my invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationof the scope of my invention as set forth in the accompanying claims.

I claim:
 1. Apparatus for providing spin rate compensation for aspinning device the output signal from which is proportional to spinrate in both frequency and amplitude and proportional to turn rate inamplitude, comprising:a compensator coupled to the spinning device, saidcompensator having an amplitude response that is inversely proportionalto frequency over the range of spin frequencies of interest. 2.Apparatus as defined in claim 1 wherein said compensator is anintegrator.
 3. Apparatus as defined in claim 1 wherein said compensatoris a low-pass filter.
 4. Apparatus as defined in claim 3, said low-passfilter having a cut-off frequency below the spin frequencies of thespinning device.
 5. In combination with a translational accelerationtransducer and means for spinning said transducer, a compensator coupledto said transducer and having an amplitude response inverselyproportional to frequency over the range of spin frequencies ofinterest.
 6. The combination of claim 5 wherein said compensator is anintegrator.
 7. The combination of claim 5 wherein said compensator is alow-pass filter.
 8. The combination of claim 7, said low-pass filterhaving a cut-off frequency below the spin frequencies of the spinningtransducer.